This invention relates to superconducting magnets in general and more particularly to a permanent current switch for short circuiting a superconducting magnet.
Once a magnetic field of a superconducting coil, particularly a high field intensity magnet coil, is generated, practically no further energy need be fed to the coil from the outside for maintaining the field, and only the energy required for the refrigeration equipment needed to maintain the superconducting state of the coil still needs to be supplied. For storing the electric energy fed into the coil, the coil can therefore be short circuited at its ends by means of a permanent current switch of minimum resistance. The current then flows almost unattenuated in the short circuited circuit so formed, and the current supply required for the excitation of the magnet coil can thereupon be interrupted.
A corresponding permanent current switch is known, for instance, from U.S. Pat. No. 4,021,633. The permanent current switch contains two contacts, of which one has, for instance, a plane contact surface, while the surface of the second contact is curved. The two contacts are made of two different materials. One part of each contact consists of a high purity metal which is normally conducting at the lowest temperatures, such as copper or aluminum, while the other part of each contact contains superconductor material. The normally conducting material serves primarily as the stabilizing material for the superconductor material. A mechanical actuating device for opening and closing the switch is designed so that the superconducting parts of the two contacts, as well as their normal conducting parts, each can be joined together directly. Therefore, special guide devices must be provided which ensure accurate contact of the superconducting surfaces. In the known permanent current switch there is further provided a vacuum chamber, in which the contacts are arranged, in order to thus preclude contamination of the surfaces. The design of the known switch is therefore relatively elaborate, and, in addition, only indirect cooling of the superconducting contact surfaces is possible.
Since the superconducting contacts of the known permanent current switch are to allow maximum current densities, but since the current carrying capacity depends on the external magnetic fields, as is well known, this switch cannot be arranged in the immediate vicinity of a superconducting magnet with high field intensity.